1. Field of the Invention
The present invention relates in general to power supplies for electronic devices, and more particularly, but not exclusively, to an apparatus to maintain redundancy in a four line cord system for use in computing environments.
2. Description of the Related Art
An N+1 redundant power system includes a number (N+1) of independent power units (such as power supplies) connected to the electrical devices, such that if one power unit fails, the system continues to function normally with the remaining N units. The power units may be power supplies, uninterruptible power supply units (UPS) or other forms of battery backup. When N=2, for example, an N+1 redundant power system has three power units. This type of power architecture provides redundancy with minimal cost and size.
High availability computer systems are expected to be powered by two independent sources, or feeds. The most common method to provide two line cord redundancy to an N+1 redundant power system is to use a transfer switch. The transfer switch connects either the first feed or the second feed, but not both, to the electrical loads.
As will be further described, where an even number of input feeds is propagated to a number of power units, a loss of a single feed will result in a loss of redundancy. In other words, where N+1 power units were originally configured, the loss of the single feed results in only N power units remaining in operation. In some cases this is due to the total power demand of the N+1 power units exceeding the supply power of the remaining feed, again as will be further illustrated.